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Formation history of the Japanese Islands
60 to 23 million years ago (Paleogene)
Fig. 9
Oligocene paleogeographic map [
]
Paleogene deposits, especially marine sediments, are distributed in
limited areas in the Japanese Islands, indicating that most of the
region to be the Japanese Islands was land during this period. The
central Hokkaido area that was a sea in the Cretaceous emerged with the
North American Plate approaching from the east. Sand and mud including
coal deposited in this uplifted area. Large coal fields were produced
in the west of the Yubari Mountains. The Yubari area was a famous for
large coal mines (almost abandoned at present). Paleogene coal fields
are also found in Tohoku (northern Honshu) and Kyushu. A rift zone was
formed with lakes and deltas in the edge of the continent from the
Oligocene to the beginning of the Miocene (after 25 million years ago).
Coal was produced in sediments of the lakes and the deltas.
The volcanic activity dormant between the Late Eocene and the Early
Oligocene became active in the Late Oligocene again. The volcanic area
expanded to the Pacific side beyond the present-day volcanic front.
23 to 16 million years ago (Early Miocene)
Fig. 10
Early Miocene paleogeographic map []
The subsidence responsible for the rift zone since the end of the
Paleogene continued in the Miocene, making lakes, marshes, and rivers
widely. The formation of the Sea of Japan began as sea water entered
into the rifted depression zone about 20 million years ago. The
Southwest Japan Arc and the Northeast Japan Arc separated and shifted
from the continental edge to the current positions with rotation.
Paleomagnetic studies revealed that southwest Japan clockwise rotated
about 45 degrees from 16 to 14 million years ago and northeast Japan
counterclockwise rotated about 46 degrees from 17 to 14 million years
ago (however, different angles and times of the rotations have been also
proposed). The Sea of Japan expanded until 15 million years ago. The Sea
of Okhotsk was also formed in the Early Miocene.
The active volcanism in the Late Oligocene was more energetic in the
Miocene; the expansion of the Sea of Japan involved intensive volcanic
activity. The volcanism on the sea floor of the Sea of Japan was
basaltic. However, dacite and rhyolite volcanic products in addition to
basaltic lava extruded on the coast of the Japanese Islands, indicating
that the Japanese Islands were dominated by tensional stress in the
Early Miocene (currently by compressive stress). Therefore, normal
faults caused abrupt collapse to form half grabens on land. An enormous
amount of volcanic products were deposited in the grabens and altered
into greenish rock. This rock is called “green tuff”, found on the
continental side of the Japanese Islands, especially in northeast Japan.
[Izu-Bonin (Ogasawara) Arc and Shikoku Basin]
The paleo-Izu-Bonin arc was located off Kyushu 25 million years ago.
This ancient arc began to split into the Kyushu-Palau Ridge and the
Izu-Bonin Arc 19 million years ago. The Izu-Bonin Arc moved eastward
with the expansion of the Shikoku Basin, which reached the present
position 15 million years ago.
16 to 11 million years ago (Middle Miocene)
Fig. 11
Middle Miocene paleogeographic map []
Northeast Japan was broadly covered with the sea between 17 and 9
million years ago. A region from Akita to Niigata (the continental side
of northeastern Honshu) and the Ou Range were long and narrow basins.
The basin of the current Ou Range area was deepest in the beginning of
the Middle Miocene. Submarine volcanic activity was active in the basin
of the Ou Range area but inactive in the Akita-Niigata basin in which
thick deposits were formed. The Tertiary formations in this region
produce a little petroleum. The submergence of northeast Japan is
accounted for which the expansion of the Sea of Japan stretched and
thinned the crust to reduce its buoyancy and east-west tension was on
the island arc for a long period.
In southwest Japan, the sea expanded into western Honshu (Chugoku
Region) to central Honshu during the formation of the Sea of Japan. The
inland became archipelagic because of the relief. Subsequently,
southwest Japan tended to emerge on the whole through into the Middle
Miocene since this region was compressed by the subduction of the
Shikoku Basin under southwest Japan resulting from the rotation during
the expansion of the Sea of Japan.
Volcanic activity occurred in Kyushu, Shikoku, and the south Kii
Peninsula through the southern part of the central Honshu (Kanto) in a
short period about 15 million years ago. The volcanism extended near the
trench, differing from common activity in the arc-trench system. This
volcanism was accounted for the subduction of the hot crust of the
Shikoku Basin heating up the island arc widely. It is known that
basaltic magma actively intruded into the Shikoku Basin.
In Hokkaido, the Kitami Mountains abruptly uplifted and a sea area with
a deep trough that developed parallel to the west of the mountains in
the middle of the Middle Miocene. The Kuril Arc began to collide with
the Northeast Japan Arc in the end of the Middle Miocene (10 million
years ago). This collision raised the Hidaka Mountains, which were a
shallow-sea area, and the Yubari Mountains.
The Izu-Bonin Arc moved and collided with Honshu by the northwestward
movement of the Philippine Sea Plate 15 million years ago. Blocks on the
Izu-Bonin Arc accreted to the Northeast Japan Arc as they could not
subside. The Misaka Mountains to the north of Mt. Fuji were a block on
this island arc. Afterward, blocks on the island arc continuously
collided and accreted to Honshu.
